1. Field of the Invention
The present invention relates in general to actuating devices and in particular to valve actuators for ball-type valves.
2. Description of the Prior Art
Ball-type valves such as valves manufactured by Orbit Valve Company of Little Rock, Ark., include a main body having a trunion-mounted ball centrally located therein for controlling fluid flow through a central passageway. In its closed position, an annular sealing surface formed on the ball is urged into sealing engagement with a sealing ring mounted in the main body. In order to move the valve to an open position, the ball is rotated 90.degree. such that a passageway formed through the ball is aligned with the passageway of the main body.
A valve stem having one end coupled to the ball is mounted within a bonnet of the valve and is utilized for controlling the operation of the valve. The stem is cammed such that axial movement of the stem produces concurrent rotational movement of the stem. Upward movement of the stem in conjunction with the concurrent rotational movement of the stem functions to pivot the sealing surface of the ball away from the sealing ring and rotate the ball 90.degree. in one direction to open the valve, while downward movement of the stem rotates the ball 90.degree. back to its closed position as a ramp surface formed on the lower end of the valve stem wedges the sealing surface of the ball tightly against the sealing ring.
While the above described type of ball valve has proved to be satisfactory, a problem may arise after the ball valve has been subjected to a period of use. This problem is caused by wear of the ramp surface provided on the one end of the stem which is utilized to wedge the annular sealing surface of the ball tightly against the sealing ring. After some time, this surface becomes worn such that, if the downward stop position of the stem is not properly adjusted, a tight seal will not be produced. Additionally, repeated usage compresses and wears the seat requiring further downward axial movement of the shaft.
A problem arises when a latching-type actuator heretofore known, is used on such a valve. The valve design necessitates that the actuator down-stop be the seating on the ball member itself. However, the aforementioned latching actuator requires that the down-stop be built into the valve, to permit the latched position to have little free play. Such free play could cause the valve to unseat. In these instances, the actuating device must typically be manually adjusted to position the downward stop of the valve stem such that a satisfactory seal will be provided. If the ball valve is located in a subterranean chamber, this adjustment is not practical. Additionally, use of such an actuator requires that this adjustment be routinely checked.